A seat equipped with a so-called three-point seatbelt (may be referred to as a seat with built-in seatbelt), into which a seatbelt and peripheral components are incorporated, is known as a vehicle seat. In this kind of vehicle seat, a manual seat reclining apparatus for a vehicle is employed for reclining the seatback relative to a seat cushion.
More specifically, the manual seat reclining apparatus for the vehicle includes a pair of upper frames and a pair of lower frames. Each upper frame has internal teeth and is mounted on the seatback. The lower frame is mounted on the seat cushion so that a pawl is attached so as to be movable in a radial direction. A cam provided around a center shaft is rotated by manual operation. Then, the internal teeth of the upper frame is engaged with the external teeth of the pawl to regulate the rotation of the upper frame relative to the lower frame, or the internal teeth of the upper frame is disengaged from the external teeth of the pawl to allow the rotation of the upper frame relative to the lower frame. Thus, the rotation of the seatback is regulated or allowed relative to the seat cushion, and the reclining angle of the seatback is adjusted or retained. It is confirmed that this type of seat reclining apparatuses withstand a large load applied to the seatback in vehicle collision or the like.
On the other hand, a seat reclining apparatus which is applicable to an electric vehicle seat includes a gear mechanism comprised of a lower frame and an upper frame. The lower frame has an internal gear and is attached to the seat cushion. The upper frame has an external gear engaging with the internal gear, and the number of the teeth in the external gear is fewer than that of the internal gear of the lower frame. The seat reclining apparatus is configured so that the rotation angle of the upper frame is successively adjusted relative to the lower frame by rotating a position where the external and internal gears are engaged (hereinafter, referred to as an engagement position). A combination of paired wedge shaped members and a cam member, interposed between the lower frame and the upper frame, is adopted as a mechanism for rotating the engaging position of the external and internal gears. The cam member is rotated by electric motor control. Then, the wedge shaped members are rotated while pressing the gears of the lower frame and the upper frame in a direction that enhances the engagement therebetween, thereby rotating the engagement position.
Data shows that the gear mechanisms provided at this type of vehicle seat reclining apparatuses are unable to withstand the large load applied to the seatback in the vehicle collision and the like. Thus, according to an embodiment of a seat reclining apparatus for a vehicle disclosed in JP 2-95304A, it is proposed that two gear mechanisms are soldered back to back so that the gear mechanisms are disposed in a mirror-image arrangement for enhancing the strength of the seat reclining apparatus.
Hence, the two gear mechanisms described above are disposed at one side of the seat relative to the seat width direction where the load applied to the seatback become large. In other words, the two gear mechanisms are located at the side where the supporting point of the seatbelt supporting the occupant shoulder is located. Further, one gear mechanism is disposed at the other side of the seat relative to the seat width direction. This configuration allows the seat reclining apparatus for the vehicle to secure the sufficient strength while preventing the size of the apparatus from becoming larger.
Meanwhile, in this type of seat reclining apparatus for the vehicle, when an electric motor is disposed thereat for rotating the engagement position of the gear mechanism (cam member), the electric motor may extend in the seat width direction and the form of the vehicle seat becomes imbalance. Consequently, the seating position of the occupant may be away from the center of the seat, leading to discomfort while sitting on the vehicle seat.
A need exists for a seat reclining apparatus for a vehicle which is not susceptible to the drawback mentioned above.